1. Field of the Invention
This invention pertains to digital data communication systems and more particularly relates to methods and apparatus for multiplexing sub-rate channels in a manner that takes maximum advantage of the switching system's switching granularity.
2. Description of the Prior Art
State of the art digital data communication switching systems are typified by the systems described in copending patent application Ser. Nos. 07/103,611, filed Oct. 1, 1987, entitled "High Speed Communication Processing System", and 07/103,612, now Pat. No. 4,852,089 also filed Oct. 1, 1987, entitled "A Digital Data Communications System". Both of these applications, hereby incorporated by reference, are assigned to the same assignee as this invention.
The system taught in the Ser. No. 07/103,612 application is defined as having m slots of data per frame and n fragments per slot to yield a total of m.times.n fragments per frame.
For example, the invention taught in the 07/103,612 application permits a T1 line to be divided up into 24 slots, each 64 Kbps wide, with 8 fragments being defined per slot (each representing 8 Kbps bandwidth). This yields 192 fragments with an 8 Kbps signalling channel left on the T1 line. Also, bandwidth allocation is contemplated for 2.048 Mbps Inter Module Links ("IMLs") within a given node, where 32 slots each 64 Kbps wide are defined (each again with eight 8 Kbps fragments).
Also taught in the referenced applications are means for allocating and deallocating bandwidth on the communication lines of the system using bit maps, how to perform allocation in a manner which minimizes call blocking, contention, etc., and how to interconnect Customer Premise Equipment (CPE) to the node oriented network via User Interfaces (UI), Network Processors (NPs), Switch Matrices (SMs), Network Interfaces (NIs), etc., via the IMLs in a given node.
The systems taught in the referenced applications support the CCITT I.463 standard for multiplexing of sub-rate channels. Also, these systems have the capability of switching fragment vs. slot packets, i.e., have an improved switching granularity as compared with the 64 Kbps switch granularity to which the I.463 standard was designed.
For the illustrative 32 slot, 8 fragments per slot IML bandwidth scheme set forth hereinbefore, I.463 mandates that sub-rate channels (defined herein as a 19.2 Kbps channel or any submultiple thereof) be assigned to 64 Kbps of bandwidth (a whole slot) before being transmitted to the switch. According to the I.463 standard, two layers of rate adaptation are performed to condition sub-rate channel data to be transmitted in the 64 Kbps packets.
Given the improved switching granularity and bandwidth allocation capabilities of systems such as those set forth in the referenced copending applications, it would be desirable, and it is an object of this invention, to improve the efficiency of bandwidth usage over the I.463 standard for sub-rate channel bandwidth allocation.
It is also desirable, and a further object of this invention, to be able to fully use the capabilities a switch, more particularly those switches taught in the above-referenced applications. Since the inventions taught in these applications make it possible to switch fragments of less than 64 Kbps, it would be desirable if sub-rate channel assignments were keyed to the granularity of the switch which in turn is ideally matched to the choice of fragment size.
For the illustrative example set out above, where the fragment size (8 Kbps) is matched to the switching granularity, an improved I.463 allocation scheme would ideally assign sub-rate channels to fragments (or integer multiples of fragments), thereby taking maximum advantage of the system's switch capacity without wasting slot bandwidth.